Metallic components, such as turbine blades in a gas turbine engine, commonly bear a thermal barrier coating (TBC) which protects the components from hot gases present in the engine. The TBC is thin, of the order of 5 to 15 thousandths of an inch. It is a ceramic-like material having a low heat transfer coefficient and having a high melting point.
The TBC protects the metal of the components from high temperatures by inhibiting heat transfer from the hot gases into the metal. That is, the metal dissipates heat rapidly while the TBC conducts heat into the metal slowly. Thus, the temperature of the metal is kept lower than that of the TBC. Therefore, the gases can be hotter, allowing a more efficient thermodynamic cycle to be used in the engine.
It is desirable to measure the thickness of the TBC during the manufacturing process. However, presently available methods of thickness measurement present difficulties. Four examples of difficulties are the following. One, direct measurement, as by sawing through a component to expose a cross section of the TBC and then measuring the TBC thickness, damages the component. Two, ultrasonic thickness measurement is not feasible because many of the TBCs are porous and disperse the ultrasonic energy. Three, computer aided X-ray tomography does not provide sufficient precision to measure the desired TBC thicknesses. Four, eddy current thickness measurement has proven to be accurate, but technical difficulties are encountered. For example, the eddy current probe typically (a pencil-like apparatus) must be kept at constant pressure and constant, known alignment with the surface of the TBC. Maintaining such alignment on complex contours such as turbine airfoils requires elaborate equipment and a highly trained operator to interpret the data.